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Full text of "The Flow Of Gases In Furnaces"

334

APPENDIX IX

lengths, with only short rubber connections between the tubes
and to the pail.) A number of very interesting experiments may
be conducted with this simple apparatus, and in this way a better
idea of the conditions governing water circulation will be obtained.
The specific heat of water and its heat capacity are unity.
The heat capacity of steam has a fractional value, but in contact
with water, under the conditions existing in a boiler tube, it has
reached the limits of its heat-absorbing capacity for the existing
temperature; that is, any additional heat imparted to the steam
in the tube will be immediately absorbed by the vaporization of a
corresponding amount of the water present. When the water is
converted into steam its volume increases from 110 to 1600 times
and it displaces from 109 to 1599 additional volumes of water.
The 4-in or 100-mm boiler tube has a sectional area of 78 cm2
54 (12.56 sq in). Its length is about 5 m 50 (18 ft) and its heating
surface 1 m2 73 (18.6 sq ft). When a boiler is driven at 200 per
cent rating it is evaporating water at the average rate of 8.1 grams
per square meter per second. On this basis each tube will evap-
orate 14 grams of water per second on the average. However, it
is not unreasonable to suppose that the lower tubes, directly
exposed to the hot gases and radiant heat from the fuel, will have
an evaporation rate of, say, ten times the average, or 140 grams
of water per tube per second. At a steam temperature of 200 C.
this would give a volume of 15,400 cm3 of steam per second. The
internal volume of the boiler tube is 43,200 cm3. This would
mean that about 35 per cent of the internal volume of the tube was
occupied by steam. At lower steam temperatures the volume of
the steam released will be much greater and fill a larger per-
centage of the tube volume. It is probable that the lower end of
the tube will be occupied by solid water, and if this is the case and
the steam is generated uniformly the full length of the tube, 70 per
cent of the area of the highest end, will be occupied by steam.
Assuming that the pitch of the tube is 30 per cent, this is suf-
ficient to give a head that will impress a velocity of 5 m 69 per
second on the steam. Friction against the water surface may be
either negative or positive, depending upon their relative veloci-
ties. Friction against the tube surface will reduce the steam
velocity.
The steam in the upper section of the tube will not be able to
absorb heat as rapidly as the water in the lower section, but as